1. Field of the Invention.
The invention relates to laser gyroscopes and particularly to laser gyroscopes which employ waves of four different frequencies within the laser gyroscope cavity. More particularly, the invention relates to output optical structures used for extracting a portion of the beam circulating within the cavity and producing therefrom output signals representing the difference in frequency between beam pairs having the same polarization within the cavity.
2. Description of the Prior Art.
In general, laser gyroscopes are devices which have two or more waves circulating in opposite directions through a laser medium so that rotation of the system will cause the round-trip time for oppositely rotating waves to differ depending upon the rate and amount of rotation. With a two-wave system, it has been found that for low rates of rotation corresponding to a small theoretical difference frequency the actual output difference frequency is zero or substantially less than would be expected due to the phenomena known as lock-in. It is believed that the lock-in problem arises because of coupling between waves which may arise from a number of possible factors including back scattering of wave energy from elements within the path such as mirrors or a Faraday rotator or from scattering centers within the laser medium itself.
The earliest attempts to compensate for this problem included one proposal in which the two beams are biased at zero rotation away from the zero output level by the use of a Faraday rotator which subjects beams propagating in different directions to different delay times. Unfortunately, simply biasing the two beams sufficiently far apart to avoid lock-in produced a large frequency difference between the two beams, so large in fact that the change in frequency caused by ordinary amounts of rotation was rather insignificant compared to the total frequency. Thus, any small drift could obliterate the actual desired signal output. Further attempts to achieve biasing included one in which the Faraday rotator was switched from one direction to another using a symmetric AC switching waveform. Such systems have proven somewhat difficult to implement since the symmetry of the AC switching waveform must be maintained to greater than one part in a million.
One of the most successful laser gyroscopes yet proposed and constructed employs four waves of two pairs each propagating in opposite directions. Such systems are shown and described in U.S. Pat. Nos. 3,741,657 and 3,854,819 to Keimpe Andringa and assigned to the present assignee, the specifications of those patents being herein incorporated by reference. In such laser systems, circular polarization for each of the four waves is used. The pair of waves, or beam, propagating in the clockwise direction includes both left and right-hand circularly polarized waves as does that propagating in the counterclockwise direction.
Two biasing components are provided. A device such as a crystal rotator produces a delay for circularly polarized waves that is different for one sense or handedness of circular polarization than for the opposite sense and is also reciprocal. That is, a wave of given polarization traveling in either direction through the crystal will be delayed by the same amount of time. Secondly, a device such as a Faraday rotator is also disposed in the wave path. Such a device is nonreciprocal, providing, for a wave of either polarization sense, a different time delay for the two directions of propagation therethrough.
In any of these laser gyroscope systems, it is necessary to extract a portion of each beam circulating within the laser cavity to produce two output signals each one of which represents the difference in frequency between wave pairs having the same sense of circular polarization within the cavity. In order to accomplish this purpose, it is desirable at some point within the output structure to combine these two beams in such a manner as to produce two new beams, each including waves having the same sense of polarization.
Previously known output structures for separating, combining, and detecting the output signals were both mechanically bulky and wasteful of signal energy and did not fully separate the polarization states resulting in crosstalk at the detector output. Because of the waste of signal energy within the output structure, larger proportions of output energy had to be extracted from the cavity requiring higher gain from the laser gain medium. The mechanical awkwardness of the structures made such systems difficult to use in many applications. Moreover, because the various components within the output optical structure were not within direct physical contact with one another, misalignment problems between the various components often arose as did drift problems. Also in many structures it was not possible to use two diodes upon a single chip for the output detectors so that the characteristics of the diodes will be nearly identical.
Accordingly, it is an object of the present invention to provide a laser gyroscope system having a mechanically rugged, compact, and efficient output structure.
It is further an object of the present invention to provide such a system in which all the components of the output structure may be directly mechanically coupled to eliminate misalignment and accompanying drift problems.
Moreover, it is an object of the present invention to provide such an output structure in which the available output signal energy is maximally utilized.
Further, it is an object of the present invention to provide a structure capable of separating completely the waves having inside the cavity the same polarization sense from the other pair of waves, thereby eliminating crosstalk between the two signal outputs of the detectors thereby resulting in a more stable and noise-free signal.
Also, it is an object of the present invention to provide an output optic structure in which two diodes on a single chip may be employed.